Tires? Alignment? Brakes dragging? 4mpg difference is a small enough gap that it could simply be down to driving habits. Constant throttle vs a hunting throttle can net you 3-4mpg right there. So even though she's driving slow, if she's feathering the throttle, out goes your mpg right there. Torque converter locking up properly? FWIW, I average 23-24mpg in my SDL, I'd expect the W210 to do better.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]